The invention relates to an anamorphic attachment for use with the basic lens system of a camera for purposes of filming and for use with the basic lens system of a projector or the like for purposes of reproduction, with a first lens subsystem of the attachment serving the main purpose of enlargement in a first direction and a second lens subsystem serving the main purpose of enlargement in a direction perpendicular or vertical to the first direction, the first subsystem containing a positive front lens group and a negative back lens group and the second subsystem a negative front lens group and a positive back lens group. Such an anamorphic attachment may be used either for purposes of filming, that is as an attachment to a camera, or for reproduction, that is as an attachment to a projection system. In both cases the attachment serves the purpose of obtaining different enlargements in two directions perpendicular to each other, in particular of the horizontal and the vertical direction. A lens group may also consist of a single lens.
U.S. Pat. No. 3,041,935 of Jacobsen shows an anamorphic attachment of the type described at the outset. The attachment is composed of two lens subsystems. The first subsystem has anamorphic properties in the first direction. The second subsystem has anamorphic qualities in the second direction that is perpendicular to the first direction. The first subsystem begins with a divergent or negative lens as front lens group and ends with a convergent or positive lens as rear lens group. The subsystem forms an afocal system. The second subsystem begins with a positive, cylindrical lens, i.e. with a convergent lens, as front lens group and ends with a negative lens as rear lens group. Also the second subsystem is afocal. Both subsystems lead to a different enlargement in the respective directions. The anamorphic factor is a result of the ratio of the enlargements in these two directions. If the front lens group of the second subsystem consists of a single lens, and is not divided into two lenses, there is no possibility of focussing. An attachment of this type may only be used for a certain distance. However, if the front lens group of the second subsystem is divided into two cylindrical lenses, focussing becomes possible. The division of the front lens group of the second subsystem is subject to the prerequisite that the air space between the two cylindrical lenses of the front lens group of the second subsystem is determined by the air space between the front lens group and the rear lens group of the first subsystem. Then, it is possible to combine the two cylindrical lenses, which are located in the same place, to a spherical lens, since their focal lengths are in accordance. The basic lens must be readjustable. The lenses of both subsystems, however, do not only have the effect in the desired direction but inevitably also in the other direction. It is very difficult to correct the resulting image distortion. Besides, correction is confined to small entrance pupils. If the diameter of the entrance pupil of the basic lens is greater than 30 mm, then the attachment may not be corrected, unless the attachment is longer than 300 mm. However, this would mean too much effort, which is why this system cannot be used for greater entrance pupils. Another disadvantage is that in the case of the embodiment which may be focussed the anamorphic factor, that is the ratio of the enlargements in the one and the other direction or the ratio of the focal lengths in the one to the other direction changes tremendously.
The change of the anamorphic factor when focussing should be as small as possible. An anamorphic attachment for a basic lens is known from DE-OS 36 29 438, where a different principle is applied. It provides for a first subsystem in which spherical lenses are used so that the characteristics in both directions, which are vertical to each other, are the same. This subsystem is composed of spherical lenses and linked to a second subsystem which eliminates the effect of the first subsystem in only one of the two directions. The advantage of this principle is that the focussing is possible by means of the anamorphic attachment located in front of the basic lens without requiring that the basic lens be readjusted. Another advantage of this principle consists in that it leads to no enlargement in one direction, i.e. there is a representation ratio of 1:1. Hence, this anamorphic attachment leads to only minor, negligible image distortion in this direction. Only the image distortion into the other direction needs to be corrected. The disadvantage of this principle is that the correction of the image distortion may only be done for small image angles.